(1) Field of the Invention
The present invention relates to a radio tag antenna structure for an optical recording medium, and a case for an optical recording medium with a radio tag antenna. Particularly, the present invention relates to a technique suitable for use to an optical recording disk medium such as a DVD, a CD or the like.
(2) Description of Related Art
Enthusiastically developed is a radio tag system wherein a reader/writer transmits a high-frequency electromagnetic wave, and a radio tag with an IC (Integrated Circuit) chip storing unique information therein receives the high-frequency electromagnetic wave, and transmits the unique information to the reader/writer. The radio tag is affixed to a book, an article of commerce or the like. The unique information on an object affixed the radio tag thereto is read out by a reader/writer. Additionaly, the unique information can be written in the radio tag. The radio tag generally comprises an IC chip and an antenna. When the radio tag receives a high-frequency signal with the antenna, a rectifier integrated in the IC chip converts the high-frequency signal into direct current components of about 3V (volt), and the IC chip performs signal processing and transmission processing with a power thereof. The frequency used for the radio tag is, for example, a 13.56 MHz band. In recent years, higher frequencies tend to be used such as a UHF band (900 MHz band), a 2.45 GHz band and the like.
A dipole antenna of a flat circuit type 110 having a half-wave length (λ/2) of the radio signal is often used as the antenna of the radio tag, as shown in FIG. 18, for example. Reference numeral 60 denotes an IC chip having the antenna 110, which performs the signal processing and transmission processing. The radio tag generally has a communicable distance of about 1 m when the transmitted signal from the reader/writer is at about 1 W (watt) although it depends on the shape of the antenna or the power consumption inside the chip. When a UHF band is used, it is known that the communicable distance can be increased from a 13.56 MHz band or a 2.45 GHz band.
As known techniques using a radio tag, there are techniques proposed in patent documents 1 to 4 below.
The technique described in patent document 1 has an object to provide an article identification mark with a burglarproofing function which can be attached to any article, and an article management system using the article identification mark. For example, an antenna of the IC chip is circularly attached about the axis of rotation of a rotary disk such as a CD, a DVD or the like, thereby realizing burglarproofing of an article such as a CD, a DVD or the like and distribution management while preventing unbalance of the rotation (irregularity in the rotation).
The technique described in the patent document 2 relates to an information recording medium and an information recording medium driving apparatus. As shown in FIG. 1 of the patent document 2, for example, a security chip storing password information therein and an antenna for the chip are directly implanted along the circumference of the axis of rotation in a part (in the inner non-recording area of a CD-ROM) of a disk of CD-ROM, an RF reader/writer of a non-contacting type having a password authenticating function is equipped to a device (CD device) which writes in and reads from the CD-ROM, and the RF reader/writer of a non-contacting type reads out a password from the security chip of the CD-ROM mounted on the CD device. Only when the password authentication is successful, the security lock of the CD device is released to allow the device to operate. Whereby, illegal copy of a music CD and the like can be prevented.
The technique described in the patent document 3 relates to an IC tag of a non-contacting type and a manufacturing method thereof. An object of the technique is to provide a tag of a non-contacting type, which does not spoil the stability of an optical disk or the like at the time of high-speed rotation even if attached to an disk-like article such as an optical disk or the like, has excellent electrical efficiency even in communications with a reader, and can stably communicate with the same. The external shape of the non-contacting IC tag is formed almost circular or oval (for example, a doughnut-like shape covering the whole surface of the optical disk), and can be attached to the surface of the optical disk.
The technique described in the patent document 4 relates to an article case with a resonant tag. An object of this technique is to make it difficult to illegally remove the resonant tag from an article case to which the resonant tag is attached as a sensor. For example, the resonant tag is fixed to a supporting plate having the same shape as the surface of a (article) containing case for a CD, the supporting plate is overlap on the case, and the containing case, the supporting plate and the resonant tag are together wrapped in this state.
[Patent Document 1] Japanese Patent Publication No. 2003-141650;
[Patent Document 2] Japanese Patent Publication No. 2000-57296;
[Patent Document 3] Japanese Patent Publication No. 2003-85502; and
[Patent Document 4] Japanese Utility Model Publication No. HEI 5-23292
When a surface (a surface on which a label is attached) on the opposite side to the information reading surface (a surface on which the laser beam is irradiated) is flawed or scratched, the reflective layer or the recording layer is apt to be damaged rather than the reading surface because the protective layer (dielectric layer) on this surface's side is very thin. For this, it is not preferable to directly attach an antenna to the label surface of a CD. As disclosed in the above patent documents 1 through 3, it is general to attach a radio tag to the inner peripheral portion where the recording layer is not formed.
However, with respect to an optical recording medium such as a DVD or the like in which a protective layer in almost the same degree as the protective layer on the information reading surface's side is also formed on the label surface's side like a DVD, attaching the antenna to the inner peripheral portion of the disk is not always the best way. Additionally, a bending work or the like of the antenna, which is not easy, is required because the area of the inner peripheral portion of the disk to which the antenna is attached is limited. An IC chip connected to the antenna has an impedance of about 500 Ω (ohm). Since a general dipole antenna has an impedance of about 72 Ω, their impedances do not match when the chip and the dipole antenna are connected as they are, which causes a disadvantage that the communicable distance is deteriorated, for example.
The above patent documents 1 through 3 disclose that a dipole antenna is formed on the inner portion of a CD or a DVD. This may cause a disadvantage that the communicable distance becomes very short or reading becomes impossible in the worst case when the antenna is connected to an IC chip for a radio tag having high impedance.